The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Plasma etching is frequently used in semiconductor fabrication. In plasma etching, ions are accelerated by an electric field to etch exposed surfaces on a substrate. The electric field is generated based on RF power signals generated by a radio frequency (RF) generator of a RF power system. The RF power signals generated by the RF generator must be precisely controlled to effectively execute plasma etching.
A RF power system may include a RF generator, a matching network and a load (e.g., a plasma chamber). The RF generator generates RF power signals, which are received at the matching network. The matching network matches an input impedance of the matching network to a characteristic impedance of a transmission line between the RF generator and the matching network. This impedance matching aids in maximizing an amount of power forwarded to the matching network (“forward power”) and minimizing an amount of power reflected back from the matching network to the RF generator (“reverse power”). Forward power may be maximized and reverse power may be minimized when the input impedance of the matching network matches the characteristic impedance of the transmission line.
In the RF power supply field, there are typically two approaches to applying the RF signal to the load. A first, more traditional approach is to apply a continuous wave signal to the load. In a continuous wave mode, the continuous wave signal is typically a sinusoidal wave that is output continuously by the power supply to the load. In the continuous wave approach, the RF signal assumes a sinusoidal output, and the amplitude and/or frequency of the sinusoidal wave can be varied in order to vary the output power applied to the load.
A second approach to applying the RF signal to the load involves pulsing the RF signal, rather than applying a continuous wave signal to the load. In a pulse mode of operation, a RF sinusoidal signal is modulated by a modulation signal in order to define an envelope for the modulated sinusoidal signal. In a conventional pulse modulation scheme, the RF sinusoidal signal typically is output at a constant frequency and amplitude. Power delivered to the load is varied by varying the modulation signal, rather than varying the sinusoidal, RF signal.
In a typical RF power supply configuration, output power applied to the load is determined by using sensors that measure the forward and reflected power or the voltage and current of the RF signal applied to the load. Either set of these signals is analyzed in a typical feedback loop. The analysis typically determines a power value which is used to adjust the output of the RF power supply in order to vary the power applied to the load. In a RF power delivery system, where the load is a plasma chamber, the varying impedance of the load causes a corresponding varying power applied to the load, as applied power is in part a function of the impedance of the load.
Further, the transition from continuous wave RF power delivery systems to pulse RF power delivery systems presents additional challenges. In a typical plasma system, the power dissipated in the plasma depends upon the impedance of the plasma. If the impedance varies in relation to the timescale of the RF pulse (typically in the range of 1 kHz-10 kHz), so as to not extinguish the plasma between pulse events, the sensors and actuators in the matching network and generator must respond on a similar timescale to provide optimal power coupling to the plasma load. Further, the time response of the impedance is plasma dependent and varies in accordance with factors such as chemistry, pressure, and power coupling. Further yet, the various parasitic elements outside of the plasma, such as resistive loss in the RF coupling antenna or the match system, present a time varying power coupling efficiency during the pulse cycle because they are a constant dissipated impedance in series with a time varying impedance load. Further yet, because the transmitted and reflected power sensors and RF generators are typically calibrated for a matched termination, power compensation due to impedance mismatch can contribute to increased variability in power delivery.
In the present, conventional control approach, the RF power supply and the matching network typically function independently. The RF power supply controls the RF frequency and power output to the matching network, and the matching network independently controls tuning of the match elements to provide an impedance match. In various conventional configurations, the impedance tuning operation is localized at the RF power supply, and the matching network generates and executes commands to perform actuation control. The impedance tuning control of conventional systems often result in competing considerations between power generation by the RF power supply and the matching function provided by the matching network.
The conventional RF control approach to treating RF power supply control and matching network control separately also presents various control complexities. For example, when attempting to achieve frequency correction by controlling impedance actuation devices in an impedance matching network, a conflicting control scenario can arise. The RF power supply attempts to regulate frequency and power while maintaining some measure of impedance tuning. Contemporaneously, the impedance matching network controls the impedance actuators to maintain a desired frequency output the RF power supply. Thus, a potential conflict arises between adjusting the RF power supply while maintaining an impedance match and, conversely, adjusting the impedance network while maintaining a desired target frequency output by RF power supply. Addressing these control complexities can enable improved RF power system control.
The challenges of such a configuration include maintaining a balance between potentially conflicting goals of frequency and power regulation and impedance tuning by the RF power supply with the often autonomous changes introduced by the impedance matching device in the impedance matching network in order to maintain a suitable match. A further challenge is introduced when measuring frequency in the impedance matching network in order to predict, but not control in a classic feedback approach, an appropriate adjustment of an impedance actuator of an impedance matching network in order to achieve the target frequency. Further, it is challenging to achieve process synchronization between frequency measurement and predicting a corresponding position of an actuator in the matching network in order to minimize the effect of impedance transient events. Further yet, realizing process repeatability and reproducibility in achieving target frequency becomes more difficult.
In present RF power generation systems, the frequency of the RF signal may be adjusted within a predetermined range about a selected target or center RF frequency in order to achieve an impedance match between the RF power generator and the load. Such frequency-based impedance tuning is referred to as automatic frequency tuning (AFT). In some AFT configurations, it is possible that the frequency of the RF signal can be adjusted towards a limit of a predetermined range of RF frequencies.